Each intervertebral disk of the vertebral column is constituted by a central element referred to as the nucleus pulposus that is enclosed in a roll of fibers referred to as the annulus. The disk interconnects two vertebral bodies and it controls movement of the vertebral column in flexing, inclination, and rotation. The disk can become damaged over time, as a result of effort, or of certain degenerative diseases, and that can give rise to the disk collapsing and/or to it functioning poorly. This can lead to various types of pathology, causing multiple pains of greater or lesser intensity and more or less severe handicaps.
This type of affection is treated by removing the unhealthy disk and replacing it either by an element that is movable or deformable or by an element that rigidly interconnects the two vertebrae in question.
Several types of prosthesis have been proposed for replacing intervertebral disks, but they are only partially satisfactory. They preserve intervertebral mobility and they restore the intervertebral distance to a value close to that ensured by a healthy disk. However in said mobility they impose particular dynamics that are not compatible with, or only partially compatible with the natural relative mobility between two vertebrae. In most situations, the prosthesis imposes dynamics specific thereto, with a center of rotation and various plane-on-plane guidances that inevitably interfere with the natural joint elements that remain between the two vertebrae, in particular the posterior articular facets. In this respect, it should be observed that the care with which a prosthesis is implanted is important since any inaccuracy in its positioning increases the severity of the conflict between the dynamics of the prosthesis and natural dynamics. This non-physiological mobility can give rise to undesirable clinical consequences. It can even be feared that there is a risk of a component migrating or of the prosthetic joint dislocating.
In addition, most known prostheses are unsuitable for restoring normal cervical or lumbar curvature. Restoring the intervertebral distance does not take account of the inclination needed for one vertebra relative to the other in the stack that leads to this curvature, where the existence of such curvature is useful for the normal biomechanics of the entire spine, and more particularly of the adjacent levels.
Furthermore, known prostheses are not adapted to absorbing impacts. A consequence of this inability, associated with the conflict between the natural dynamics and the dynamics of the joint, can lead to premature wear both of natural elements and of prosthetic elements, thereby running the risk of degrading the clinical state of the patient.
An intervertebral disk prosthesis known in particular from WO 2007/057555 comprises:                a rigid top plate;        a rigid bottom plate; and        an elastically compressible intermediate cushion housed between the inner surfaces of the two plates, the assembly having the feature of being subdivided in the thickness direction into two units resting one on the other via complementary contact surfaces. That structure is in the form of a part that does not impose any constrained connection between the two vertebrae it connects together (naturally for amplitudes that correspond to natural relative movements). As a result, the natural guides of such relative movement remain preponderant (in particular the posterior articular facets) and their integrity is preserved. In addition, by appropriately selecting the shape and the state of the contact surfaces, it is possible to control the nature and the concentration of the stresses and the deformations that exist at said surfaces.        